Pulse doppler and navigation system

ABSTRACT

A vessel navigation system employs gated transmitting and receiving transducers for developing pulsed Doppler frequency shifted signals indicative of velocity components along selected orthogonal axes. Gated feedback arrangements are employed to produce continuous oscillations having the same frequency as the Doppler shifted signals. The signals are converted to digital form, and processed to yield, inter alia, velocity, drift angle, and distance information. The information is corrected to compensate for variations in the acoustical propagating characteristic of the ocean medium, and provision is made for obtaining reliable data under severe rolling conditions.

United States Patent 1 1 1111 3,719,923

Waterman 5] March 6, 1973 [54] PULSE DOPPLER AND NAVIGATION 3,065,46311/1962 Turner ..343/9 SYSTEM 2,908,888 10/1959 Kirkland. ...340/33,436,721 4/l969 Farr 340/3 [75] Invent: l Nble waerman Salt Lake3,496,524 2/1970 Stavis et al. .340/3 C1ty, Utah [73] Assignee: EdoWestern Corporation, Salt Lake Prim ry Xdm nerRiChard A. Farley City,Utah AttorneyDavid V. Trash and William S. Britt [22] Filed: Sept. 11,1969 [57] ABSTRACT [21] Appl' 857l70 A vessel navigation system employsgated transmitting R n d us, A li i D t and receiving transducers fordeveloping pulsed Doppler frequency shifted signals indicative ofvelocity components along selected orthogonal axes. Gated feedbackarrangements are employed to produce con- [63] Continuation-impart ofSer. No. 818,893, April 24,

1969, Pat. No. 3,594,716.

[52] U C] i I 340/3 D 343/9 tinuous oscillations having the samefrequency as the 51 Int. Cl ..cois 9/66 Dppler.shifted Signals- TheSignals are muted 581 Field of Search ..340/3 3 D- 343/8 9 7 A digitalfmm, and Pmessed Yield inter alia,

velocity, drift angle, and distance information. References Cited Theinformation is corrected to compensate for varia- UNITED STATES PATENTStions in the acoustical propagating characteristic of the ocean medium,and provision is made for obtain- 3,49l,333 1/1970 Goulet et al ..340/3ing reliable data under severe rolling conditions. 1,864,638 6/1932Chilowsky..... ...340/3 3,257,638 6/1966 Kritz et al ..340/3 35 Claims,23 Drawing Figures I, ammo JNCLINOMETER ca. L; 1;,

-?/5 22 0 mil/811A 200 2/5 Avmafi/ 5 226 K 25 GA TE 5 yALUE AVERAGE flz"ZZZ?" News ans 230 l 232 GATE 24 GATE s FREQUENCY mummy I s cou'rmuousamen sum-rm OUTPUT PATENTEUHAR 61975 SHEET 3 OF 9 :omu

INVENTOR. GLENN NOBLE WATERMAN BY k4,, 6w; ATTORNEYS PATENTEDHAR BIQYSSHEET 8 OF 9 INVENTOR. GLENN NOBLE WATERMAN A TTORNE Y5 PATENTEUHAR61975 719,923

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INVENTOR.

GLENN NOBLE WATEPMAN A rroR/vs 5 PATENTEDHAR 61975 3,719,923

SHEET 8 OF 9 FIGBA FIG.8B T1 j n U v FIGBC F'IGBD FIG. 8E

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GLENN NOBLE WATERMAN wfw PULSE DOPPLER AND NAVIGATION SYSTEM Thisapplication is a continuation-in-part of my copending application Ser.No. 818,893, filed Apr. 24, 1969 now US Pat. No. 3,594,716.

This invention relates to electronic navigation apparatus and, morespecifically, to a pulsed system employing the Doppler apparentfrequency shift principle for measuring velocity components of a ship,and quantities derived therefrom.

It is desirable that personnel operating a ship at sea know the velocitycomponents of the vessel along two orthogonal coordinate axes, e.g.,fore-aft and port-starboard for the direction of actual travel, or withrespect to a desired course and along an azimuth normal thereto. Inaddition to the velocity information which is useful per se, otherquantities may be computed from the determined velocity values, such asdistance traveled and/or distance to go, and long and short term driftangles with respect to a desired course.

Moreover, this measured and derived velocity and position informationbecomes especially important in some applications requiring very precisenavigation. Such applications include, for example, underwaterexploration and survey, and military precise pattern travel as for minesweeping or laying.

It is an object of the present invention to provide an improved marinenavigation system.

More specifically, an object of the present invention is the provisionof a pulsed wave Doppler navigation system which employs gated feedbackcircuitry for maintaining continuous replicas of returnedfrequencyshifted signals, and digital circuitry for accurately andreliably computing instantaneous velocity components ofa vessel alongselected orthogonal axes.

Another object of the present invention is the provision of a Dopplernavigation system which computes and displays distance and drift angleinformation derived from measured velocity data.

Still another object of the present invention is the provision of aDoppler navigation system which includes apparatus for readily selectingcoordinate axes along which vessel velocity is to be determined.

Yet another object of the present invention is the provision of aDoppler navigation system which is operative under severe ship-rollingconditions, and also operative in extreme water depths.

The above and other objects of the present invention are realized in aspecific, illustrative navigation system employing two essentiallyindependent channels for measuring and displaying vessel velocity alongtwo selected orthogonal axes. Each channel includes a time sharedtransducer for transmitting repetitive pulse bursts of sinusoidal energydirected along an associated sensing axis and, sometime thereafter, forgenerating an electrical output responsive to a returned portion of thetransmitted signal reflected by the ocean bottom, or by some otherscattering medium. If the ship has any motion along the axis beingexamined, there will be an apparent increase or decrease in frequency ofthe reflected signal.

The returned signal is supplied to a gated feedback loop which generatesat its output a continuous periodic signal of a frequency correspondingto that of the returned Doppler frequency shifted pulses. The feedbackcircuitry is rendered operative by a strobe signal during the midportion of the returned signal.

A measure of the emitted signal and the continuous replica of thereturned pulses are mixed, converted to digital form, and processed toidentify the particular direction of motion, if any, along each axis.Circuitry is provided to develop a sequence of pulses characterized by arepetition rate dependent upon the apparent Doppler frequency shift. Acounter is employed to count the number of velocity-indicating pulsesoccurring within a repetitive time gate period which is corrected forvariations in the temperature and salinity of the water, theseparameters being factors in the frequency shifting process. The pulsesaccumulated during the periodically recurring time gate intervals are adirect measure of absolute axial velocity, and are converted toappropriate output displays which may be spaced about the ship.

Similarly, the number of cycles of the continuous wave, corrected forpropagation variations in the water medium, are counted and displayed toyield the distance traveled along the sensed axis. Correspondingly,displays are provided for velocity and distance for the other axis.Drift angle information is developed by employing a trigonometricoperation on the orthogonal distances (long term drift) or velocities(instantaneous drift).

The above and other objects, features and advantages of the presentinvention are realized in a specific, illustrative embodiment thereof,presented hereinbelow in conjunction with the accompanying drawing, inwhich:

FIGS. 1 through 6 depict an illustrative pulsed Doppler navigationsystem embodying the principles of the present invention;

FIGS. 7A-7G depict the voltage waveforms associated with strobe pulsegenerating circuitry in the system of FIGS. l-6;

FIGS. 8A through 8F are timing diagrams depicting the voltage waveformsassociated with digital computation circuitry in the system of FIGS.1-6;

FIGS. 9A and 9B are orthogonal views respectively depicting fore-aft andport-starboard energy radiation and reflection paths for the pulsedDoppler system'of FIGS. l-6;

FIG. 10 illustrates one embodiment of a transducer array employed in thesystem of FIGS. 1-6; and

FIG. 11 depicts the spatial organization of FIGS. 1-6.

In accordance with the principles of the present invention, the velocityof a ship along selected orthogonal axes, e.g., fore-aft andport-starboard is developed by digitally processing a regeneratedcontinuous wave replica of pulsed Doppler apparent frequency shiftedsignals. To this end, a transducer (FIG. 9A) is employed under the shipand oriented to periodically radiate energy forward and downward alongthe fore-aft axis. The radiated energy may illustratively comprisepulsed bursts of a sinusoidal wave having a relatively high frequencysuch as k.c. to

ensure operation for all types of sea floors, and with agood depthcapability. A part of the emitted energy is scattered and reflected bythe ocean bottom, or by any other scattering medium such as a stationarywater mass, thermal boundary, algae particles or the like. Thetransducer 100, when operated in a receiving mode, receives a portion ofthe reflected energy, and converts it into electrical form.

As is well known, there is an apparent frequency shift of a signaltransmitted between two points having a relative motion therebetween. Inthe instant applica tion, the frequency of the signal received by thetransducer 100 (f,) will be higher than that emitted by the transducer(f,,) if the ship has a forward velocity component, and smaller than f,if the ship is moving astern. In particular, the Doppler frequency shiftf,-f is directly proportional to the ships forward velocity andinversely proportional to the propagational speed of the emitted andreturned wave through the water. Thus, by proper scaling of the system,the frequency difference f,.f, yields a direct measure of the shipsfore-aft speed when the system is corrected for variations in theacoustical wave propagation speed of water in the manner discussedbelow.

Similarly, there is a transmitting-receiving transducer 180 (FIG. 9B)oriented to starboard and downward for measuring a velocity componentorthogonal to the alignment of the transducer 100, this beingport-starboard for the assumed situation.

The transducers 100 and 130 may comprise individual units orthogonallyaligned as described above or, alternatively, two transducer elementsmounted on a single pod as shown in FIG. 10. For the pod array, thetransducers 100 and 180 are mounted at right angles to each other onsloping surfaces about the lower portion of the pod.

Referring now to FIGS. l-6, arranged as in FIG. 11, there is shown anelectronic navigation system for developing the desired velocity andother information employing the above-described Dopplerfrequencyshifting principles. The arrangement includes an oscillator 145(FIG. 1) for selectively imparting sinusoidal energy to the for-aft andthe port-starboard transducers 100 and 180, it being assumed at thispoint that data along, and normal to the direction of actual ship'smotion is desired. Examining the signal flow and processing associatedwith fore-aft velocity, which is illustrative of the computationaloperations for the other speed measurement, a gate circuit 148 isperiodically opened to pass pulsed sinusoidal energy from the oscillator145 to the transducer 100 by way of a power amplifier 150. The gate 148is opened for the duration of a transmit pulses which repetitivelyappear at the 1 output of a one shot multivibrator 158 as discussedbelow. During these energy radiating, or transmitting periods, therelatively low output voltage at the output of the one shotmultivibrator 158 gates off (blanks) a signal receiving amplifier 110which cannot supply an output signal during pulse radiation.

The sinusoidal energy burst emitted by the transducer 100 is directedforward toward the ocean bottom, and is in part reflected back andrecovered by the transducer 100 sometime later after the transmit periodterminates. Depending upon the motion of the ship, the signal frequencygenerated by the transducer 100 is either greater than, less than, orthe same as that which it previously emitted.

The recovered pulsed return signal is supplied by the transducer 100 tothe amplifier 110 through a limiting net work 105 which further protectsthe amplifier 110 against damage and extraneous noise by limiting thepeak signal values it will pass. The limiting network 105 may comprisebipolar voltage clamps or the like.

The amplifier 1 10 is gated on during the receiving interval by therelatively high voltage at the one shot multivibrator 158 0 outputterminal following each transmit pulse. The amplifier output, comprisingDoppler frequency shifted sinusoidal bursts, is supplied to a thresholdcircuit 140, e.g., a Schmidt trigger, where the amplitude of the signalsis examined. If the signals are insufficient in amplitude to switch thethreshold circuit 140, they are termed spurious noise signals and nofurther circuit action takes place. If true signals are being processed,the threshold circuit develops at its output bursts of square waveformswhich correspond in number and in frequency to the input pulsedsinusoidal wave.

The interrupted square wavetrain output of the threshold circuit 140 issupplied to the gated feedback loop of FIG. 2, and also supplied tocircuitry (FIG. 1) which generates timing signals for controlling theincidence of the transmit pulses, and for developing a strobe signal forperiodically enabling the FIG. 2 feedback loop about the center of thereceived return pulses for best system performance.

Examining first the timing circuitry of FIG. 1 in conjunction with theassociated waveforms of FIGS. 7A-7G, the transmitted sinusoidal burstsare depicted in FIG. 7A, and the corresponding reflected and receivedbursts in FIG. 7B. A ramp generator 172 is employed to selectivelyproduce a relatively slow ramp (solid waveform 700 in FIG. 7C followinga time a therein) when its output is unclamped from ground by a gate166, e.g., when a transistor becomes nonconductive. A second rampgenerator provides a relatively rapid ramp function (dashed waveform 701in FIG. 7C following a time b therein) when its output is unclamped by agate 164, as when a transistor 165 is not conducting. The output fromthe ramp generators 170 and 172 are supplied to a comparator(difference) circuit 174 which triggers a one shot multivibrator 176when the output voltage from the fast ramp generator 170 exceeds that ofthe slow ramp generator 172.

Taking as a starting point in a typical timing cycle a transmit pulse atthe 1" output of monostable multivibrator 158, this pulse persists froma time 0 to a time a (FIG. 70), and the radiated sinusoidal pulse (FIG.7A) has similar time bounds. At the end of this pulse (time a), the lowvoltage at the 1 output of multivibrator 158 shuts off the gate 166,allowing the generator 172 to develop the slow ramp, as shown for thewaveform 700 in FIG. 7C following time a.

The reflected signal is received during the period b-e of FIG. 7B, and apeak detector 160 generates a voltage waveform corresponding to thepositive envelope of the received signal as shown by the dashed-dottedcurve 703 in FIG. 7B. This waveform is negated by an element 162 whichthereby shuts off gate 164 for the interval b-e thus allowing the outputof ramp generator 170 to rise (waveform 701 in FIG. 7C following thetime b).

The output voltage of generator 170 increases in voltage faster thanthat of the generator 172, and exceeds the output of the generator 172at the time c at or near the middle of the received pulse. Accordingly,the comparator 173 switches state from time c (FIG. 7D) until the end ofthe received pulse at time e. The output of the comparator 174 (FIG. 7D)triggers a monostable multivibrator 176 which is timed to supply anoutput pulse for the interval c-d of FIG. 7E.

The output of multivibrator 176 occurs about the center of the receivedpulse (FIG. 7B), and is used as a strobe signal to enable the FIG. 2feedback loop as discussed below.

To complete a timing cycle of operation, the pulse output from themultivibrator 176 is delayed by a unit 152 the output of which triggersthe one shot multivibrator 158 via an OR logic gate 156 to initiate anew transmit pulse, and thereby also a new cycle of operation. Finally,a ramp generator 154 and a threshold circuit 155 are coupled between the1 output of the multivibrator 158 and the OR gate 156 to initiate afirst cycle of operation, and to initiate successive cycles if a triggerpulse is somehow suppressed or otherwise not forthcoming, e.g., if noreturned signal is received or the timing loop interrupted. The rampgenerator 154 is normally reset by the multivibrator 158 before it cantrigger the threshold circuit 155, and thus has no effect when thesystem is operating in the normal cycling mode described above.

The pulsed burst of square wave signals developed by the thresholdcircuit 140 each receiving cycle are supplied to the gated feedbackcircuit of FIG. 2, along with the strobe pulses at the output of themultivibrator 176 which occur at the center portion of the square waves.The purpose of the FIG. 2 gated feedback circuitry is to constrain avoltage controlled oscillator 240 to emit a continuous, noninterruptedsinusoidal signal having the same frequency as the square waves from thecircuit 140 which are supplied only on a pulsed basis.

The strobe signal is employed to improve system accuracy andreliability. In particular, as the propagation distance from the ship tothe reflecting layer increases, a corresponding increase is effected inthe width of the radiated beam as it strikes the reflecting layer.Accordingly, the duration of the return pulse (the interval b-e in FIG.73) increases since all parts of the intersection of the radiatedacoustical beam with the reflecting surface contribute in some measureto the return signal. As clear from the waveform 700 in FIG. 7C, theoutput of the ramp circuit 172 attains an increasingly higher potentialas the interval between the emitted and returned pulse increases.Accordingly, it takes the output of the fast ramp generator 170 anincreasingly long time, measured from the beginning of a return signalto reach the time c where the strobe pulse begins. Thus, the arrangementof FIG. 1 generates the strobe pulse at or near the center of thereceived pulse, where data is best obtained e.g., for accuracy andoptimum signal strength, independent of the water depth with itscorresponding variation in return pulse width.

The strobe pulse from the multivibrator 176 opens a gate 200 to pass thecenter portion of the pulse burst from the threshold circuit 140 to theinput, error v sensing portion of the gated frequency tracking loop and,in particular, to the set input terminal ofa J-K flipflop 215. Thefeedback loop operates in three distinct modes under control of threegates 234, 244 and 246. For the entire duration of the strobe pulse, thegate 234 is open operatively connecting the control input of the voltagecontrolled oscillator 240 into an active feedback loop state, while thegate 244 is held off by an inverter 232 to disconnect the control inputof the oscillator 240 from the output of a sample and hold circuit 250.

At the beginning of the strobe period, with only the gate 234 open, theoutput of the flip-flop 215, a square wave asymmetrical for the generalcase is passed to an average value detector 225, e.g., a low passfilter. The detector 225 supplies an output to a forward gain amplifier226 which is proportional to the average value of the output waveform ofthe flip-flop 215. This, in turn, depends upon the off vis-a-vis onintervals for the active flip-flop output device, i.e., depends upon theamount and polarity of the output asynchronism. The output of theamplifier 226 comprises a voltage which, when applied to the oscillator240 via the gate 234, is of a value to operate the oscillator at thesame frequency as the input signal applied to the flip-flop setterminal.

For steady state operation when the input and output frequencies arealike, the input signal repetitively sets the flip-flop 215 and theoutput of the oscillator 240, applied to the clock, or toggle input,resets the flip-flop. The relative phasing of the two signals isautomatically adjusted to develop the proper on-off periods for theflip-flop output to develop the required oscillator control voltage. Asthe input frequency changes, the on-off periods (in general asynchronousor nonsymmetrical) of the flip-flop changes to vary the oscillator 240control voltage to track the incoming frequency, the response of thefeedback loop and its dynamic range being dependent upon its forwardgain transfer characteristics, frequency characteristic and the like.Accordingly, for the full duration of the strobe pulse, the oscillator240 directly follows the incoming square wave in frequency.

A short while after the beginning of the strobe pulse and as determinedby a one shot circuit 228, an AND gate 230 is fully switched and opensthe gate 246 for the remainder of the strobe interval. This connects theamplifier average value output of the amplifier 226 (effectively thecontrol voltage required to set the oscillator 240 to the properfrequency) to an average value network 248 having a large reactiveinertia (long time constant), and which changes output potential slowly.Accordingly, the network 248 will slowly track the output of theamplifier 226 each cycle and, by reason of its inertia, provide anoutput voltage to the sample and hold circuit 250 which essentiallyrepresents the average of the most recently encountered control voltageoutputs of the amplifier 226.

Following the end of the strobe pulses for the interval betweenconsecutive such pulses, both gates 246 and 234 are closed and the gate244 opens. The output potential of the sample and hold 250 thus controlsthe oscillator 240 during this time, supplying it with a control voltagewhich is the average of the dynamic control voltages impressed on theoscillator 240 by the amplifier 226 during the more recent strobeintervals. Accordingly, the output frequency of the oscillator 240between strobe pulses comprises the average of the incoming square wavefrequency during these more recent strobe periods. The response of thesample and hold path is made slow since the large inertia of a shipprevents rapid speed changes, and marked return frequency changes cannotoccur.

In summary then regarding the FIG. 2 frequency tracking circuitry, theoscillator 240 supplies a continuous oscillation which corresponds infrequency to'that of the incoming square 'waves which are present foronly repetitive short time intervals.

The continuous sinusoid at the output of the oscillator 240, embodyingthe Doppler frequency shifted information, is supplied as one input to amixer 316 (FIG. 3), and also passes via a 45 phase shifting lead network320 to a mixer 318. The output from the radiated signal sourceoscillator 145 is directly supplied as a second input to the mixer 316,and supplied to an input port of the mixer 318 via a 45 degree phase lagnetwork 322. Since the frequencies supplied 'to the lead and lagnetworks 320 and 322 are constrained to .a narrow range,

these networks may simply comprise passive resistive and capacitiveimpedances. Filters 324 and 328 are connected to the outputs of themixers 316 and 318', and comprise low pass filters to select and passtherethrough only the first'order difference frequency of the many sumand difference products generated by the non-linear mixers 3'16 and 318.Thus, the outputs of the filters 324 and 328 are sinusoids of a Dopplerdifference frequency f,.f,, which are a direct measure of the fore-aftvelocity component of the ship. However, the phase of the output'signalsfrom the filters 324 and 328 differs, with the sinusoid at the output ofthe filter 328 'lagging'the output of the filter 324 by 90 electricaldegrees if f, exceeds f (vessel moving ahead), and leading by 90 iffexceeds f (vessel heading astern).

square wave supplied by the circuit 326. The phase shifted square waveoutput of the squaring circuit 330 (FIG. 8C for motion ahead and FIG. SEfor motion astern) is directly supplied as a second input to the ANDgate 412 and via an inverter 414 to a second input of the AND gate 416(FIGS. 8D-ahe'ad and 8F- astern). r

If the ship is moving ahead, the AND gate 412 will be fully energizedand switched once each square wave cycle'since both of its inputs willbe at a relatively high voltage condition during the relatively highvoltage sampling output of the one shot multivibrator 410 (FIGS. 8B and8C). For this condition, the AND gate.

supplied to a second input of the AND gate 420 via a A delay element411, and also to an AND gate 422. With. the flip-flop 418'in the setcondition, a delayed replica of the output from the one shotmultivibrator 410 is developedonce each cycle at the Doppler differencefrequency rate. These pulses fully enable. the gate 420 V which thus hasa like pulse train at the output thereof.

. Thus, the outputof the AND gate 420 comprises a The outputs from thefilters 324 and 328 are respectively supplied to squaring circuits 326and 330 which transform the input sinusoidal waveform to digital, or

' square'wave form'for subsequent digital processing.

The squaring circuits may comprise, for example, threshold Schmitttrigger embodiments. The nonphaseshifted, reference output from thesquaring circuit 326 is shown in FIG. 8A and the output of the squaringcircuit 330 is shown in FIG. 8C (90 phase lag for motion ahead) and 8E(90 degrees phase lead for motion astern).

The frequency of the square wave present at the output of the squaringcircuit 326 (or the equivalent square wave frequency at the output ofthe element 330) is a direct measure of the Doppler frequency shift, andthus of ship's speed. A pair of headphones 344 may be connected underoperator control by a switch 342 to the circuit 326 through an audioamplifier 340. The pitch of the audio reproduced by the phones 244supplies a quick and easily discernible indication of the ships for-aftspeed to an operator primarily focusing his attention elsewhere, e.g.,upon piloting activities.

The audio indication approximates the absolute foraft velocity of theship, uncorrected for changes in the ocean wave propagatingcharacteristic, and without supplying forward vis-a-vis aft information.Circuitry 409 (FIG. 4) is provided to digitally identify a ships motionas being ahead or astern, and includes a one shot multivibrator 410which is adapted to supply a pulse (FIG. SE) at each trailing edge ofthe square wave supplied by the squaring circuit 326. The pulse outputof the one shot multivibrator 410 partially enables two AND gates 412and 416 during a relatively short sampling interval once during eachcycle of the sequence of pulses having a repetition rate which is adirect measure of the velocity of the ship. in the forward direction.

With the ship'moving ahead, the flip-flop 418 is set and the unenergized0" output thereof disables the AND'gate 422 which is thus inhibited fromswitching.

When the ship moves astern, the'output of the squaring circuit 330lags'the reference wave from the circuit 326 by (FIG. 8E). Accordingly,when this condition obtains, the gate 416, supplied with the invertedreplica of the phase advanced square wave (FIG. 8F), is fully enabledand drives the flip-flop 418 to a reset condition. The 1" and O outputsfrom the flip-flop 418 respectively disable and partially enable the ANDgates 420 and 422, thus permitting only the AND gate 422 to pass therearward velocity indicating pulses supplied by the one shotmultivibrator 410 and the delay 411. Hence, pulses are developed at theoutput of the AND gate 422 when the ship is moving astern and at theoutput of the AND gate 420 when the ship is moving ahead.

The Doppler rate of the signals at the output of the gate 420 or 422provides a measure of velocity front and rear. The number of suchpulses, being in essence the integral of the rate, provide a directmeasure of distance travelled front and rear.

The outputs from the AND gates 420 and 422 are supplied to circuitry 423which is provided for reliability considerations, i.e., to require thattwo consecutive pulses be supplied from the output of either the gate420 or 422 before a fore vis-a-vis aft" indication is displayed.Assuming that the vessel is traveling ahead, repetitive pulses appear atthe output of the AND gate 420 and not at the output of the gate 422. Aflip-flop 424 is provided to count the output pulses from the AND gate420 through a normally enabled AND gate 423 and, responsive to thesecond such pulse, the flipflop 424 sets a flip-flop 428. The output ofthe set flip-flop 428 thus exhibits a low voltage which blocks switchingof an AND gate 434 thereby inhibiting any spurious aft-motion indicatingpulses spuriously developed at the output of the AND gate 422 fromreaching an OR gate 436 and subsequent processing apparatus. Further,the low voltage at the 0 output of flip-flop 428 disables an AND gate425 to effectively isolate erroneous pulses from the verifier anddirection designating circuitry 427. Thus, responsive to the requisitetwo output pulses from the gate 420, spurious rearward velocity pulsesare inhibited from having any electrical effect.

The AND gates 423 and 425 are adapted to reset the pulse countingflip-flops 426 and 424, respectively, if the incoming pulses alternatebetween fore and aft indications. Once the requisite two consecutivedirection signaling pulses have been received, one of the gates 423 or425 is inhibited from any further effect by the low voltage output fromthe flip-flop 430 or 428.

The flip-flop 428, like a corresponding flip-flop 430, initially residesin a reset condition. Thus, the 0 output from the flip-flop 430 isinitially high and partially enables an AND gate 432 which thereforepasses all forward velocity indicating pulses generated at the output ofthe AND gate 420. These forward motion pulses are passed through the ORgate 436 for eventual counting, and velocity and distance measurement.

correspondingly, if the boat is moving astern, a flipflop 426 counts twooutput pulses from the gate 422 via a normally operative AND gate 425and sets the flipflop 430, thereby disabling the AND gate 432 frompassing any spurious forward motion indicating pulses. Further, the 0output of the flip-flop 428 remains high and partially enables the ANDgate 434, thereby passing the astern velocity indicating pulses from thegate 422 to and through the OR gate 436. The set flipflop 430 alsoblocks the AND gate 423 to cut off the flow of any spurious forwardpulses developed by the gate 420.

The output of the OR gate 436 comprises a sequence of pulses having arepetition rate which identifies the velocity component of the vesselalong the sensed axis, whether forward or astern. An increasingrepetition rate for pulses supplied by the OR gate 436 identifiedincreased ships speed (either forward or astern), and vice versa.

The direction of travel of the ship is indicated by an illuminated oneof two lamp groups 452 or 454 which are respectively energized by lampdrivers 448 or 450 when the ship is traveling fore or aft. One pair oflamp groups 452 and 454 are included in each output display. Control forthe lamp drivers 448 and 450 is derived from the l outputs from theflip-flop 428 and 430 acting through a memory 440, with the l output ofthe flip-flop 428 being high if the ship is traveling ahead and the 1output of the flip-flop 430 being high if the ship is traveling asternas discussed above. The two-stage latched memory 440 receives as inputsthe outputs of the flip-flops 428 and 430, and also a control Signalwhich selectively constrains the memory 440 to operate in a sample(track) or hold (store) mode. More specifically, the outputs of thelatched memory 440 will follow and reproduce the data input signals whenthe memory is in a sample mode (a high sample input control voltage) andwill retain the last sampled input voltages when the control signalbecomes a relatively low (hold) voltage. The outputs from the memory440, in turn, illuminate only one of the lamp groups 452 or 454depending upon the state of the flip-flops 428 and 430 at the criticalcontrol voltage transition from a sampling to a hold voltage level.

The velocity magnitude indicating pulses present at the output of the ORgate 436 pass through a normally enabled AND gate 510 (FIG. 5) to apulse frequency dividing scaler net work 512 which illustratively comprises a digital counter having a signal output from the mostsignificant digit stage thereof. In accordance with standard knowntechniques, a plurality of parallel-connected switches 514 through 514,are employed to alter the counter capacity when any one or more of theswitches is closed. The switches 514 are normally distributed with theoutput information displays at various points about a vessel to providefor control at any of these points.

The variable scaler 512 is adapted to reduce the rate of Dopplerdifference frequency pulses to provide an output display in conventionalengineering units (knots, feet per second or the like). As discussedbelow, the output display is generated by counting the number of dividedpulses which occur in a corrected, cyclical time span, e.g.,approximately one second. Thus, the scaler 512 wired for a first countcapacity may be operative to divide down the Doppler differencefrequency such that each pulse at its output may effectively represent aforward-aft velocity of one foot per second for the nominal one secondgating interval or, with a switch 514 closed to change the scaler countcapacity, a speed of one-tenth of a knot. The pulse output of the scaler512 is supplied to the count input of a digital counter 516, typicallyof a binary coded decimal configuration.

Circuitry 605 for developing periodic sample/hold and CLEAR signals tocyclically examine (sample) and reset the counter 516 for converting thecontinuum of Doppler difference pulse into a velocity indicating cyclicaccumulation is shown in FIG. 6. The timing circuitry 605 of FIG. 6 isoperative in conjunction with the count capacity of the scaler 512 toprovide a counter pulse accumulating interval such that the peak countstate of the counter 416 supplies a direct indication of a particularcomponent of ships velocity. This gating interval has some nominalvalue, e.g., one second, which is corrected to offset the inversedependence of the Doppler difference frequency upon the propagationvelocity of sound in water. Accordingly, the circuitry of FIG. 6 isadapted to take the salinity and temperature of the ocean into accountsince these are the principal factors in the change of acousticalvelocity in the fluid medium. More specifically, the circuitry 605normalizes the gating interval to render the navigation system of FIGS.1 through 6 operative and accurate to any medium propagationcharacteristic.

An oscillator 610 supplies a sinusoidal output to a mixer 620, with asecond input for the mixer comprising the output of a voltage controlledoscillator, 618. Variable resistors 614 and 616 are employed to vary thecontrol voltage input to the voltage controlled oscillator and, therebyalso, to vary its output frequency. The variable resistors 614 and 616may be manually operated by dialing in measured or known temperature andsalinity values or, alternatively, these resistors may comprisetransducers having impedances or output voltages which vary withparameters actually sensed. Further, the input of the voltage controlledoscillator may be derived from a velocimeter which directly supplies anoutput voltage which varies with the acoustical propagating speed of theocean.

The mixer 620, operative in conjunction with a low pass filter 622,produces a sinusoidal output voltage having a frequency which has anominal value to give rise to the normal gating interval (e.g., onesecond) after further processing, but which has a frequency correctionto account for deviations in the ocean propagation characteristic from apredetermined norm. A squaring circuit 624 converts this extractedsinusoidal signal to digital form, and the resulting square wave signalpasses to a scaling, or pulse dividing net work 626. The scaler 626reduces the pulse repetition rate to the desired corrected gatinginterval, e.g., approximately 1 second. That is, the output from thescaling circuit 626 is adapted to comprise an output pulse which sets aflip-flop 628 about once each second acting through a differentiator629.

The zero output terminal of the flip-flop 628 returns to a low voltagecondition when the flip-flop becomes set, thereby generating a negativegoing transient which triggers a one shot multivibrator 633 forproducing one pulse each second which is slightly delayed from theoutput of the scaler 626. This output pulse, illustratively comprising atransition from a low voltage to a relatively high voltage and backagain, gives rise to two independent circuit operations. First, thispulse is supplied to the latched memories 518 (FIG. and 440 (FIG. 4) tooperate these memories in a track or sample mode. While tracking, thesememories attain and internally store the digital values of the inputsignals supplied thereto, with these stored signals being present at theoutput of the memories 440 and 518. When the sample" output pulse fromthe one shot multivibrator 633 terminates, the memories 440 and 518 arereturned to their normal hold state, with the signals stored during thesampling interval being retained at the output of the memories until thenext sample level pulse is received.

The output from the one shot multivibrator is also supplied to anadditional delay producing one shot multivibrator 630 which produces aCLEAR output pulse a fixed time after the sample level pulse has beenproduced. This CLEAR pulse resets the flip-flop 628 until the nextfollowing gating pulse is received from the scaler 626; resets theflip-flops 428 and 430; and resets the velocity magnitude counter 516 toinitiate a new axial velocity measuring operation.

The velocity measurement stored in the memory 518 (velocity magnitudebut not direction) at the end of each cyclic sampling interval isdisplayed by a plurality of decade displays 526 through 526, which maybe disposed at various positions about the ship, i.e., in the controlroom, on the bridge, and elsewhere. The output from the memory 518 issupplied to a plurality of decoder-drivers 520 which perform a dataconversion function which depends upon the requirements of the displays526. The decoder-drivers 520 are well known to those skilled in the artand may comprise, for example, apparatus for converting binary codeddecimal information to lamp display driving one out of seven or one outof ten codes. The latched memory 440 gives rise to the requisitedirectional information by illu- 5 minating either one of the lampgroups 452 or 454 to respectively indicate whether the ship is headingfore or aft.

With the above-described circuitry in mind, an illustrative velocitymeasuring sequence of circuit operation for the navigation system ofFIGS. 1-6 will now be described in conjunction with the above-consideredfore-aft motion channel which is illustrative of the other(port-starboard) channel 185 as well.

The multivibrator 158 repetitively opens gate 148, thereby energizingthe transducer 100 which emits pulse bursts of sinusoidal acousticalenergy forward and downward toward a reflecting medium. The energy issupplied to the transducer by the oscillator 145, the gate 148, and apower amplifier 150. A portion of the radiated energy is reflected backto and recovered by the transducer 100, and amplified by the amplifier110 which is operative after a blanking interval corresponding to thepulse transmission period.

The ramp generator 172 develops a slowly rising ramp following thetransmission period, and the ramp generator 170 produces a relativelyfast rising ramp at the beginning of a received pulse. The comparator174 and the one shot multivibrator 176 generate a strobe pulse at thecenter of each received pulse. A replica of the strobe pulse, delayed bythe unit 152 by a sufficient interval to complete processing of eachreceived signal, triggers the multivibrator 158 via the OR gate 156 toinitiate a following cycle.

The ramp generator 154 and threshold circuit 155 are provided to ensurethat transmit periods will recur if a return signal is not received. Theelement 154 is normally periodically reset by the output of themultivibrator 158 before it can trigger the threshold circuit 155, andperforms no system function when return signals are properly beingprocessed.

The received signal pulse bursts are distinguished from noise in athreshold circuit 140 and supplied with the strobe pulses to thefrequency tracking gated feedback loop of FIG. 2. The output of thevoltage controlled oscillator 240 in FIG. 2 is a continuous sinusoidhaving the same frequency as that of the individual cycles in the pulsebursts. The gate 234 is open for the duration of the strobe pulse tocomplete a fast response feedback loop such that the oscillator 240operates exactly in frequency with the coincidentally supplied inputpulse.

During the latter portion of the strobe pulse, the gate 246 opens, and anetwork 248, having a slow response, tracks the oscillator 240 controlvoltage at the output of the amplifier 226 in the fast feedback loop.Between strobe pulses, the gate 244 opens, and the sample and holdcircuit 250 maintains the oscillator 240 at a continuous outputfrequency essentially corresponding to the time average of thefrequencies of the most recent input pulse bursts.

Also, the fixed and voltage controlled oscillators 610 and 618 generaterecurring velocity measuring interval gating pulses which, after digitalprocessing, are produced at the output of the one shot circuit 633. Themixer 620 receives the output of the oscillator 610 and the output ofthe voltage controlled oscillator 618 which effects a salinity andtemperature water propagation speed correction, and the selected firstorder mixer output frequency is converted to digital form by thesquaring circuit 624. The frequency output of the squaring circuit 624is divided down by the scaler 626 which repetitively sets the flip-flop628 at approximate one second intervals corrected for variations in theocean wave propagation speed. The negative voltage transients at theoutput of the flip-flop 628 are coupled to the one shot multivibrator633 which responds thereto by supplying positive going data samplingpulses having a relatively short time duration and a repetition rate ofapproximately one cycle per second. The sampling pulses are supplied tothe latched memories 440 and 518 to temporarily operate these memoriesin the sample mode, quickly returning to the normal hold mode. Thus, thelatched memory 440 samples directional information (fore or aft)approximately once each second and, coincidentally therewith, the memory518 stores the velocity magnitude information manifested by the contentsof the counter 516.

The velocity directional and magnitude information (and distancetravelled and other information as discussed below) is developed bymixing the output of the oscillator 145, which is a replica of theemitted signal, with the output of the controlled oscillator 240, whichis a continuous representation in frequency of the reflected pulsesreceived by the transducer 100. These two signals are mixed withoutrelative phase shift in the mixer 316 and with 90 relative phase shiftin the mixer 318. The like Doppler difference frequency outputs from themixers 316 and 318 are converted to digital form by squaring circuits326 and 330, and the resulting square wave outputs, having a 90 phaseshift therebetween are supplied to the circuitry 409 for substantiallydetermining direction information. This directional identification isaccomplished by steering the Doppler difference frequency to the outputof the AND gate 420 if the ship is moving forward, or to the output ofthe AND gate 422 if the ship is moving astern. This signal steering, inturn, is effected by sampling the phase shifted square wave output ofthe circuit 330 during a fixed point of each reference square wave, thegate 412 being operative to set the flip-flop 418 if the ship is movingforward (the phase shifted output of the circuit 330 being positiveduring the critical sampling instant). Alternatively, the gate 416resets the flip-flop 418 (the output of circuit 330 being negative atthe critical sampling instant) if the ship is moving astern. The ANDgate 420 passes delayed pulses, corresponding in frequency to theDoppler difference frequency, through the AND gate 420 after theflipflop 418 is set (a high voltage on the flip-flop l output terminal)and the gate 422 passes this Doppler difference frequency information ifthe 0 output of flipflop 418 exhibits a relatively high output voltage.

Circuitry 423 verifies that two consecutive forward motion indicatingpulses or two consecutive rearward indicating pulses are received and,responsive to one of these conditions obtaining, locks up the circuitry423 for the duration of the sampling cycle. If the vessel is identifiedas going forward, a gate 432 is continuously enabled to pass velocitymetering pulses for measurement via the OR gate 436. If the boat ismoving astern,

the AND gate 434 passes the difference frequency information through ORgate 436.

Both of the gates 432 and 434 are initially enabled by the 0" outputs ofthe flip-flops 428 and 430 at the beginning of a velocity measuringcycle. Similarly, the AND gates 423 and 425 are normally enabled by theO outputs of these flip-flops. When two consecutive inputs are impressedon one of the leads to the verifier circuit 423, the correspondingcounting flip-flop 424 or 426 sets an associated flip-flop 428 or 430,thereby impressing a low voltage at the 0 output of the flip-flop. Thislow flip-flop 0 output voltage disables one of the AND gates 432 or 434and disables a corresponding one of the AND gates 423 or 425 to isolateand block any spurious pulses indicative of motion opposite to theactual direction of travel of the vessel.

The flip-flop 428 is set (a high l output voltage) if the vessel ismoving forward and the flip-flop 430 is set if the vessel is movingastern. The memory 440 examines these flip-flops during the criticalsampling interval, and between sampling times, illuminates the lampgroup 452 or the group 454 to indicate that the vessel was detected asmoving either forward or astern during the most recent samplinginterval.

The pulses at the output of the OR gate 436, indicative of velocitymagnitude but not direction, are normally passed through the AND gate510 and are divided down by the scaler 512. The capacity of the scaler512 is adjusted by particular settings of the switches 514 such that theoutput of the scaler supplies pulses having a significance measured by astandard engineering unit. For example, if a Doppler differencefrequency of 300 cycles per second corresponds to a forward speed of oneknot, the scaler might advantageously have. a capacity of 30 states todivide the input pulses by a factor of 30, such that each output pulsesupplied to the counter 516 represents a velocity for the normal 1second gating interval of one-tenth of a knot. The switches, which mightbe more than one in number, can make several adjustments. For example,the scaling factor may be changed to vary the significance of eachoutput pulse from one-tenth of a knot to one foot per second.Alternatively, the capacity could be changed to effect a division bythree such that each output pulse would identify th ofa knot.

The output pulses which occur during the approximate 1 second gatinginterval since the counter 516 was reset by the last CLEAR pulse aresuccessively stored in the counters 516. The counter 516 is normallydisconnected from the displays, since the hold level control signaloutput of multivibrator 633 constrains the latched memory 518 to presentto the decoder drivers 520 only the velocity information derived duringthe last sampling interval. At the end of the approximate one secondgating interval, the control input of the memory 518 momentarily changesto the sample mode and the contents of the counter 516 are registered inthe latched memory 518. At this time, the relatively low sample/holdcontrol signal blocks the AND gate 510 from passing any further velocityaugmenting pulses. The latched control then returns to its normal holdstate when the one shot multivibrator 633 times out, and the contents ofthe counter 516, directly indicative of the velocity component understudy, is decoded and displayed by the units 526.

Shortly after the sample/hold signal returns to normal, a CLEAR pulse isdeveloped by the one shot multivibrator 630 which clears theabove-enumerated flipflops and the counter 516 to initiate a newvelocity measuring operation. In the gating interval between samplingpulses, the velocity direction and magnitude last computed are displayedby the lights 452 or 454, and by the displays 526.

Thus, the above-described digital data processing apparatus has beenshown by the above to reliably measure and display the direction andmagnitude of ships velocity along its fore-aft axis. Circuitry identicalto that described above is included in the control and display circuit185 associated with a sensing axis orthogonal to that for apparatusconsidered in detail above, i.e., abeam of the vessel for the assumedcase.

The above-described mode of operation for the navigation system of FIGS.1-6 may be practiced where a convenient scattering surface exists toreflect a fair portion of the emitted signal back to the transducer 100.Where no such layer exists, e.g., for some very deep water bodieswithout stratified water masses, a switch 222 may be closed, such that areplica of the transmit pulse from the multivibrator 158, delayed by avariable delay 220, opens the gate 200 after each transmit pulse. Anyreflected energy returned to the transducer 100 during this pseudostrobe pulse interval, and large enough to pass through the thresholdcircuit 140, is fed to the frequency tracking feedback loop of FIG. 2.Such energy may be reflected from algae, or other particle matter in thewater, with the delay of unit 220 being adjusted to obtain the bestpossible data. The remainder of the system circuitry operates as beforeto yield the velocity measurements from the continuous output of thefrequency tracking loop, except that the circuit members 154 and 155provide its recurring system timing.

Also, reliable measurements may be derived in conditions of heavy seas,where the vessel is rolling (and/or pitching) considerably. When thisenvironmental condition obtains, a switch 215 (FIG. 2) is closed therebyconnecting the output of a threshold circuit 205 to the gate 200. Aninclinometer 210 supplies a voltage to the threshold circuit 205 whichis a function of its vertical orientation and, thereby also, of theship's vertical orientation. When the ship is other than in a verticalor near vertical position, the inclinometer 210 and the thresholdcircuit 205 supply an output which inhibits opening of the gate 200.Accordingly, at such times, the frequency tracking feedback loop isinterrupted, and the controlled oscillator 240 output is maintained bythe sample and hold circuit 250.

When the ship rolls through its vertical or near vertical position, theoutput of the inclinometer 210 switches the circuit 205 thereby removingthe gate 200 inhibiting signal acting through switch 215. Accordingly,the gate 200 operates as described above to update the frequencytracking loop if required during a strobe pulse interval.

In addition to measuring velocity along the two perpendicular sensingaxes, the navigation system of FIGS. l-6 includes structure formeasuring the net distance travelled along each of the two axes.Moreover, the distance computation is corrected for variations in thepropagation characteristic of the ocean medium.

It will be recalled that a square wave, embodying Doppler frequencyshift information, was steered through the AND gate 420 when the vesselmoves ahead, and through the AND gate 422 when the vessel moves astern.Also, the repetition rate of the square wave pulses incorporates adirect measure of velocity in the given direction. Therefore, since thepulse rate identifies velocity, each individual pulse represents a givendistance of travel. For example, if a Doppler frequency of 200 cyclesper second corresponds to a ship velocity of 10 feet per second, eachcycle (square wave pulse) represents 10/200 feet or 005 feet. Each pulseoutput from the AND gate 420 would represent 0.05 feet travelled ahead,and each pulse from the AND gate 422 would represent 0.05 feet movedastern.

If the acoustical velocity of the ocean increases, the Doppler shifteddifference frequency decreases and, accordingly, the flip-flop 644remains set for a greater part of the 1 second interval to permit theproper number of pulses (which are occurring at a slower repetitionrate) to pass through the appropriate gate 460 or 462. Conversely, adecrease in the propagation velocity gives rise to a longer reset statefor the flip-flop 644 such that the correct number of pulses (whichoccur more frequently) will pass to the counter 466. Thus, the number ofpulses actually passing through a gate 460 or 462 for any given shipsvelocity will remain the same for changing acoustical propagation speedsfor the ocean.

The forward distance indicating pulses generated by the AND gate 460 aresupplied to a scaler 461 which counts the pulses down to a desiredengineering unit (mile, feet, or the like). Correspondingly, therearward distance pulses from the AND gate 462 are divided down in acorresponding scaler 463. These scalers 461 and 463 may be made variableto vary the engineering unit employed as for the scaler 512.

The scaler 461 supplies forward distance pulses to the up count inputterminal of an up-down counter 466, and the scaler 463 is connected tothe down" counter input terminal. As the vessel moves ahead, the countstate of the counter 466 increases as the scaler 461 exercises the upterminal, while the count state decreases when the boat moves astern asthe scaler 463 pulses the down" input terminal. Thus, the count state ofthe counter 466 at any time identifies the net distance travelled alongthe associated (fore-aft) sensing axis. This information is decoded to aseven or ten line code in one or more decoders 468, and is displayedabout the ship in one or more displays 470.

The control and display circuitry for the orthogonal channel includessimilar apparatus to that described above for measuring the net distancetravelled along the axis associated therewith.

To effect a fore-aft distance measurement, the outputs from the ANDgates 420 and 422 are supplied to AND gates 460 and 462 along with anormally high, enabling timing signal from the distance timing circuitry635 of FIG. 6. This timing signal normally partially enables the ANDgates 460 and 462, and periodically blocks the gates with a low voltagefor a time duration which corrects for the varying acousticalpropagation characteristic of the ocean.

The ocean velocity variations are taken into account by the distancetiming circuitry 635 of FIG. 6. Basically the 1 output of a flip-flop644 is high to enable the AND gates 460 and 462 for most of a repetitivegating interval, e.g., one second. To this end, the output of areference oscillator 640 (or of the oscillator 610) is scaled down infrequency by a divider 642 to pulse the set input of the flip-flop 644once each second, thereby developing a relatively high output potentialat the l output of the flip-flop 644.

Also, the output of the squaring circuit 624, quantities comprises asquare wave having a repetition rate which varies with the oceanvelocity characteristic as discussed above, is divided down in frequencyby a scaler 646 which has a viz., capacity less than that of the scaler642. The output of the scaler 646 periodically continuously theflip-flop 644 to temporarily block the distance pulse passing AND gates460 and 462 until the next following output of scaler 642, which resetsthe scaler 646 while again setting the flip-flop 644.

The above-discussed has assumed that velocity and distance informationhas been desired with respect to the fore-aft and port-starboard axes ofthe ship. These quantities may be desired for any other orthogonal axissystem, e.g., along a desired course and normal to the course. The axissystem may be selected in either of two ways, viz., by physicallyrotating the transducers of transducer pod so that the transducers 100and 180 continuously face their corresponding axis; or bytrigonometrically operating on the output signals derived for a fixedaxis system (e.g., fore-aft and portstarboard) in accordance with thewell known axis rotation equations, as by a digital or analog general orspecial purpose computer.

An arrangement for physically rotating the axes system is shown in FIG.1 and comprises a differential generator 130 which is supplied with theelectrical output of the ships gyrocompass 115 (indicative of actualships heading), and with the setting of a shaft 120 which is set toselect a desired axis system. The shaft setting varies the couplingbetween theinput and output of the generator 130, and the generator 130supplies angular difference signals to a synchro follower 135 or otherfollower motor system which adjusts the orientation of the transducers100 and 180 to the azimuth specified by the angle of the shaft 120. Manyother organizations well known to those skilled in the art will alsosuffice for adjusting the orientation of the transducer pod to a desiredposition as specified by a mechanical or electrical setting.

Finally, the instantaneous drift angle of the vessel may be determinedby performing a trigonometric operation (e.g., arc sin or arc tan) onthe two velocity components. This may be effected by converting thedigital contents of the two velocity registers (counter 516 and itscounterpart in the second channel) into analog voltages in adigital-to-analog converter. The two analog signals are then supplied toany of the well known units for computing either function.

sin- (b/a) =tan (b/a) or the approximation c (b/a) (small angle),

where c represents the output drift angle, and a and b are the twoanalog velocity signals. Correspondingly,

identical structure can compute the long term drift angle by operatingon the two axial distance measurements, i.e., the contents of theregister 466 and its second channel counterpart. In addition, totaldistance travelled may be derived by commonly available apparatus forcomputing the relationship where d is the total distance, and a and bare analog voltages representing the distance travelled along the twosensed axes.

It is to be understood that the above-described arrangement is onlyillustrative of the principles of the present invention. Numerousmodifications and adaptations thereof will be readily apparent to thoseskilled in the art without departing from the spirit and scope of thepresent invention. For example, energy can be transmitted downward andoutward in both directions along a given axis, and the inverse frequencyshifted return signals beat together in the mixers 216 and 218. Thisessentially doubles the output Doppler difference frequency, henceincreasing system resolution and accuracy, and offsetting pitch and rolleffects.

What is claimed is:

1. In combination in a pulsed Doppler navigation system, transducermeans for radiating emitted energy downward and outward along a desiredsensing axis and for recovering a reflected portion of said emittedpulsed energy, said returned energy portion having a frequency dependentupon the speed of said transducer means along said sensing axis; gated,normally open, feedback loop means for generating a continuous periodicoutput signal of a frequency related to that of the recovered pulsedenergy; and means for gating said feedback loop closed when a recoveredsignal is available for processing.

2. A combination as in claim 1 further comprising a source ofoscillations to be emitted by said transducer means, and circuit meansresponsive to the difference in frequency between the output of saidoscillation source and the continuous periodic output signal of saidfeedback means for computing the velocity of said transducer means alongsaid sensing axis.

3. A combination as in claim 2 wherein said feedback loop meanscomprises a voltage controlled oscillator, error signal means responsiveto the output of said voltage controlled oscillator and to said returnedpulse energy for producing a potential for controlling said oscillator,first gating means for selectively connecting said error signal meansand said voltage controlled oscillator, control voltage storage means,and second gating means selectively connecting said control voltagestoring means to said voltage controlled oscillator.

4. A combination as in claim 3 wherein said control voltage storingmeans comprises a sample and hold circuit, and wherein said gatedfeedback loop means further comprises third gating means selectivelyconnecting said error sensing means and said sample and hold means.

5. A combination as in claim 4 wherein said error sensing meanscomprises a flip-flop having two input terminals thereon respectivelysupplied with said returned pulsed signals and they output of saidvoltage controlled oscillator, and average value detector meansconnected to the output of said flip-flop.

6. A combination as in claim further comprising slow response couplingmeans connected intermediate said third gating means and said sample andhold means.

7. A combination as in claim 6 wherein said flip-flop included in saiderror sensing means comprises a J-K flip-flop having set and clock inputterminals, said returned signals being connected to said set flip-flopinput terminal and the output of said voltage controlled oscillatorbeing connected to said clock input terminal.

8. A combination as in claim 2 further comprising means for disablingsaid feedback loop means when the vertical orientation of said systemdeparts substantially from vertical.

9. A combination as in claim 8 wherein said vertical departure disablingmeans includes gate means for inhibiting said feedback loop means,inclinometer means for providing an output signal which characterizesthe relative vertical orientation of said system, and circuit means fordisabling said gating means when the output of said inclinometer meansdeparts from a vertical signaling condition.

10. A combination as in claim 2 further comprising means for rotatingsaid sensing axis with respect to a vessel in which said combination ismounted.

11. A combination as in claim 2 further comprising means for obtainingDoppler frequency shift velocity information along two orthogonal axes,and wherein said transducer means includes two transducer elementsdisplaced at right angles on a single transducer pod.

12. A combination as in claim 2 wherein said feedback loop gating meanscomprises means for gating said feedback loop means into an operativecondition about the midpoint of said returned energy pulses recovered bysaid transducer means.

13. A combination as in claim 12 wherein said feedback loop gating meanscomprises a relatively fast ramp generator, a relatively slow rampgenerator, means for enabling the output of said slow ramp generatorfollowing transmission of a pulse of emitted energy, additional gatingmeans for enabling said fast ramp generator at the beginning of arecovered energy pulse, and comparator means for generating a strobegating pulse for rendering said feedback loop mean operative when theoutput of said fast ramp generator attains a predetermined relationshipwith the output of said slow ramp generator.

14. A combination as in claim 13 further comprising timing meansconstraining said navigation system to sequentially radiate energy pulsebursts including transmit gate means intermediate said source ofoscillations and said transducer means, and means responsive to eachoutput of said comparator means for enabling said transmit gate means.

15. A combination as in claim 14 further comprising a relaxationoscillator, threshold means for selectively enabling said transmit gatemeans, and means for selectively resetting said relaxation oscillatorresponsive to a pulse generated by said comparator means.

16. A combination as in claim 2 wherein said computing circuit meanscomprises means for generating first and second digital square wavesignals each having a frequency reflecting the difference in frequencybetween the emitted and returned signals and having a relative phasedependent upon the direction of motion of said transducer means alongsaid axis, means for sampling said second square wave signal during apredetermined point in the cycle of said first square wave signal, meansresponsive to the output of said sampling means for presentinginformation identifying the relative direction of said transducers alongsaid sensing axis, means for developing a sequential pulse traindependent in frequency upon the frequency of one of said square waves,digital counter means for counting the pulses produced by said pulsedeveloping means, information outputting means, latching memory meansfor operating said information outputting means in accordance with thecontents of said counter during a sampling interval, and time gate meansfor periodically initializing said counter to a reference state and forperiodically supplying control signals to said latching memory means.

17. A combination as in claim 16 wherein said time gate means includesmeans for varying said counter initializing periodicity and said controlsignal supplying periodicity responsive to variations in the acousticalpropagating characteristic of the emitted energy propagating medium.

18. A combination as in claim 17 wherein said time gate means includes areference oscillator, means for supplying a signal having a frequencydependent upon the acoustical propagating characteristic of the wavepropagating medium, mixer and filter means for obtaining an outputsignal dependent upon the difference in frequency between the output ofsaid reference oscillator and the output of said propagationcharacteristic dependent signal supplying means, and scaler means forperiodically supplying pulses responsive to a plurality of inputoscillation cycles supplied thereto by the output of said mixer andfilter means.

19. A combination as in claim 18 wherein said time gate means furtherincludes a flip-flop connected to said scaler means, said scaler meansbeing adapted to selectively set said flip-flop, and delay meansconnected to an output of said flip-flop for resetting said flip-flop.

20. A combination as in claim 16 wherein said first and second digitalsquare wave generating means comprise first and second mixer and filtermeans, squaring circuit means connected to the output of each of saidinput filters, means for supplying said emitted and returned signals tosaid first and second mixer and filter means, and means connected at theinput of said second mixer and filter for producing a relative phaseshift between said emitted and said returned signals.

21. A combination as in claim 16 further comprising scaler meansconnected intermediate said digital counter means and said pulsedeveloping means.

22. A combination as in claim 21 further comprising switching means forvarying the pulse frequency division factor for said scaler means.

23. A combination as in claim 22 wherein said second square wavesampling means includes means for generating an output pulse responsiveto a voltage transient condition for said first square wave signal,first and second AND gates each having two inputs, said voltagetransient responsive pulse being supplied to one input of each of saidAND gates, means for sup-

1. In combination in a pulsed Doppler navigation system, transducermeans for radiating emitted energy downward and outward along a desiredsensing axis and for recovering a reflected portion of said emittedpulsed energy, said returned energy portion having a frequency dependentupon the speed of said transducer means along said sensing axis; gated,normally open, feedback loop means for generating a continuous periodicoutput signal of a frequency related to that of the recovered pulsedenergy; and means for gating said feedback loop closed when a recoveredsignal is available for processing.
 1. In combination in a pulsedDoppler navigation system, transducer means for radiating emitted energydownward and outward along a desired sensing axis and for recovering areflected portion of said emitted pulsed energy, said returned energyportion having a frequency dependent upon the speed of said transducermeans along said sensing axis; gated, normally open, feedback loop meansfor generating a continuous periodic output signal of a frequencyrelated to that of the recovered pulsed energy; and means for gatingsaid feedback loop closed when a recovered signal is available forprocessing.
 2. A combination as in claim 1 further comprising a sourceof oscillations to be emitted by said transducer means, and circuitmeans responsive to the difference in frequency between the output ofsaid oscillation source and the continuous periodic output signal ofsaid feedback means for computing the velocity of said transducer meansalong said sensing axis.
 3. A combination as in claim 2 wherein saidfeedback loop means comprises a voltage controlled oscillator, errorsignal means responsive to the output of said voltage controlledoscillator and to said returned pulse energy for producing a potentialfor controlling said oscillator, first gating means for selectivelyconnecting said error signal means and said voltage controlledoscillator, control voltage storage means, and second gating meansselectively connecting said control voltage storing means to saidvoltage controlled oscillator.
 4. A combination as in claim 3 whereinsaid control voltage storing means comprises a sample and hold circuit,and wherein said gated feedback loop means further comprises thirdgating means selectively connecting said error sensing means and saidsample and hold means.
 5. A combination as in claim 4 wherein said errorsensing means comprises a flip-flop having two input terminals thereonrespectively supplied with said returned pulsed signals and the outputof said voltage controlled oscillator, and average value detector meansconnected to the output of said flip-flop.
 6. A combination as in claim5 further comprising slow response coupling means connected intermediatesaid third gating means and said sample and hold means.
 7. A combinationas in claim 6 wherein said flip-flop included in said error sensingmeans comprises a J-K flip-flop having set and clock input terminals,said returned signals being connected to said set flip-flop inputterminal and the output of said voltage controlled oscillator beingconnected to said clock input terminal.
 8. A combination as in claim 2further comprising means for disabling said feedback loop means when thevertical orientation of said system departs substantially from vertical.9. A combination as in claim 8 wherein said vertical departurE disablingmeans includes gate means for inhibiting said feedback loop means,inclinometer means for providing an output signal which characterizesthe relative vertical orientation of said system, and circuit means fordisabling said gating means when the output of said inclinometer meansdeparts from a vertical signaling condition.
 10. A combination as inclaim 2 further comprising means for rotating said sensing axis withrespect to a vessel in which said combination is mounted.
 11. Acombination as in claim 2 further comprising means for obtaining Dopplerfrequency shift velocity information along two orthogonal axes, andwherein said transducer means includes two transducer elements displacedat right angles on a single transducer pod.
 12. A combination as inclaim 2 wherein said feedback loop gating means comprises means forgating said feedback loop means into an operative condition about themidpoint of said returned energy pulses recovered by said transducermeans.
 13. A combination as in claim 12 wherein said feedback loopgating means comprises a relatively fast ramp generator, a relativelyslow ramp generator, means for enabling the output of said slow rampgenerator following transmission of a pulse of emitted energy,additional gating means for enabling said fast ramp generator at thebeginning of a recovered energy pulse, and comparator means forgenerating a strobe gating pulse for rendering said feedback loop meanoperative when the output of said fast ramp generator attains apredetermined relationship with the output of said slow ramp generator.14. A combination as in claim 13 further comprising timing meansconstraining said navigation system to sequentially radiate energy pulsebursts including transmit gate means intermediate said source ofoscillations and said transducer means, and means responsive to eachoutput of said comparator means for enabling said transmit gate means.15. A combination as in claim 14 further comprising a relaxationoscillator, threshold means for selectively enabling said transmit gatemeans, and means for selectively resetting said relaxation oscillatorresponsive to a pulse generated by said comparator means.
 16. Acombination as in claim 2 wherein said computing circuit means comprisesmeans for generating first and second digital square wave signals eachhaving a frequency reflecting the difference in frequency between theemitted and returned signals and having a relative phase dependent uponthe direction of motion of said transducer means along said axis, meansfor sampling said second square wave signal during a predetermined pointin the cycle of said first square wave signal, means responsive to theoutput of said sampling means for presenting information identifying therelative direction of said transducers along said sensing axis, meansfor developing a sequential pulse train dependent in frequency upon thefrequency of one of said square waves, digital counter means forcounting the pulses produced by said pulse developing means, informationoutputting means, latching memory means for operating said informationoutputting means in accordance with the contents of said counter duringa sampling interval, and time gate means for periodically initializingsaid counter to a reference state and for periodically supplying controlsignals to said latching memory means.
 17. A combination as in claim 16wherein said time gate means includes means for varying said counterinitializing periodicity and said control signal supplying periodicityresponsive to variations in the acoustical propagating characteristic ofthe emitted energy propagating medium.
 18. A combination as in claim 17wherein said time gate means includes a reference oscillator, means forsupplying a signal having a frequency dependent upon the acousticalpropagating characteristic of the wave propagating medium, mixer andfilter means for obtaining an output signal dependent upon thedifference in frequency between the output Of said reference oscillatorand the output of said propagation characteristic dependent signalsupplying means, and scaler means for periodically supplying pulsesresponsive to a plurality of input oscillation cycles supplied theretoby the output of said mixer and filter means.
 19. A combination as inclaim 18 wherein said time gate means further includes a flip-flopconnected to said scaler means, said scaler means being adapted toselectively set said flip-flop, and delay means connected to an outputof said flip-flop for resetting said flip-flop.
 20. A combination as inclaim 16 wherein said first and second digital square wave generatingmeans comprise first and second mixer and filter means, squaring circuitmeans connected to the output of each of said input filters, means forsupplying said emitted and returned signals to said first and secondmixer and filter means, and means connected at the input of said secondmixer and filter for producing a 90* relative phase shift between saidemitted and said returned signals.
 21. A combination as in claim 16further comprising scaler means connected intermediate said digitalcounter means and said pulse developing means.
 22. A combination as inclaim 21 further comprising switching means for varying the pulsefrequency division factor for said scaler means.
 23. A combination as inclaim 22 wherein said second square wave sampling means includes meansfor generating an output pulse responsive to a voltage transientcondition for said first square wave signal, first and second AND gateseach having two inputs, said voltage transient responsive pulse beingsupplied to one input of each of said AND gates, means for supplyingsaid second square wave in inverted relationships to a second input ofsaid first and second AND gates, an output from one of said AND gatessignaling motion in one direction along said sensing axis and an outputfrom the other of said AND gates signaling motion in the other directionalong said sensing axis.
 24. A combination as in claim 23 wherein saidsampling means further comprises third and fourth AND gates, means forsupplying one of said square wave signals to each of said third andfourth AND gates, a flip-flop selectively set by said first AND gate andselectively reset by said second AND gate, one output from saidflip-flop being connected to said third AND gate and the other output ofsaid flip-flop being connected to said fourth AND gate.
 25. Acombination as in claim 24 wherein said relative direction identifyingmeans comprises verifier means for signaling the relative direction onlyafter the incidence of a plurality of pulses corresponding to motion ina like direction.
 26. A combination as in claim 25 wherein said verifiermeans includes first and second additional counting means respectivelyconnected to said third and fourth AND gates, first and second verifierflip-flops selectively set by said first and second additional counters,additional latched memory means for developing a relative directiondisplay dependent upon the relative states of said verifier flip-flops,and means for periodically disabling said verifier circuit for furtherchange after one of said verifier flip-flops is set.
 27. A combinationas in claim 16 wherein said information outputting means comprises atleast one decade display, and at least one decoder-driver meansresponsive to the output of said latching memory means for selectivelyactivating said decade display.
 28. A combination as in claim 27 furthercomprising additional receiving and transmitting transducers forproducing relative frequency signals indicative of motion along at leastone additional sensing axis, and means connected to said transducers forcomputing and displaying the direction and magnitude of motion alongsaid additional sensing axis.
 29. A combination as in claim 16 furthercomprising means for audibly reproducing one of said square wavesignals.
 30. A combination aS in claim 2 further comprising means forcomputing the distance travelled by said system along said sensing axis.31. A combination as in claim 30 wherein said distance computing meansincludes means for developing a digital square wave train having afrequency reflecting the difference in frequency between the emitted andreturned signals, means for determining the relative direction of motionof said transducer means along said sensing axis, an up-down counterhaving up and down count input terminals, first and second gating meansfor connecting said square wave train to said up and down counter inputterminals, respectively, said relative direction determining meansincluding means for selectively enabling only one of said first andsecond gating means.
 32. A combination as in claim 31 further comprisingmeans for correcting said distance computing means for variations in thecharacteristics of the propagation medium.
 33. A combination as in claim32 wherein said correcting means comprises an oscillator, a flip-flop, ascaler connecting said oscillator to said flip-flop, variable frequencymeans having an output oscillation frequency dependent upon thepropagation characteristic of said propagation medium, additional scalermeans connecting said variable frequency means with said flip-flop,means connecting the output of said first scaler to initialize saidsecond scaler, and further gating means for selectively isolating the upand down counter input terminals of said up and down counter responsiveto signaling from said flip-flop.
 34. A combination as in claim 2further comprising means for computing the velocity of said vessel alongtwo orthogonal sensing axes, and means responsive to said orthogonalvelocity computations for computing the instantaneous drift angle ofsaid system.